A typical electronic image sensor is comprised of an array of a large number of very small light detectors, together called a xe2x80x9cpixel arrayxe2x80x9d. These sensors typically generate electronic signals representative of each pixel that have amplitudes that are proportional to the intensity of the light received by each of the detectors in the array. In electronic cameras imaging components produce an optical image of a scene onto the pixel array. The electronic image sensor converts the optical image into a set of electronic signals. These electronic cameras typically include components for conditioning and processing the electronic signals to allow images to be converted into a digital format so that a digital processor can process the images, which can then be stored, transmitted digitally and or displayed. Various types of semiconductor devices can be used for acquiring the image. These include charge couple devices (CCD""s), photodiode arrays and charge injection devices. The most popular electronic image sensors utilize arrays of CCD detectors for converting light into electrical signals. These detectors have been available for many years and the CCD technology is mature and well developed. One big drawback with CCD""s is that the technique for producing CCD""s is incompatible with other integrated circuit technology such as metal oxide semiconductor (MOS) and complementary metal oxide semi-conductor (CMOS) technology, so that processing circuits for the CCD arrays must be produced on chips separate from the CCD""s.
Another currently available type of image sensors is based on MOS technology or CMOS technology. These sensors typically have multiple transistors within each pixel. The most common CMOS sensors have photo-sensing circuitry and active circuitry designed in each pixel cell. They are called active pixel sensors (APS""s). The active circuitry consists of multiple transistors that are inter-connected by metal lines; as a result, the area occupied by the circuitry is opaque to visible light and cannot be used for photo-sensing. Thus, each pixel cell typically comprises photosensitive and non-photosensitive circuitry. In addition to circuitry associated with each pixel cell, CMOS sensors have other digital and analog signal processing circuitry, such as sample-and-hold amplifiers, analog-to-digital converters and digital signal processing logic circuitry, all integrated as a monolithic device. Both pixel arrays and other digital and analog circuitry are fabricated using the same basic process sequences.
Small cameras using CCD sensors typically consume large amounts of energy (as compared to cameras with CMOS sensors) and require high rail-to-rail voltage swings to operate CCD. This can pose problems for today""s mobile appliances, such as Cellular Phone and Personal Digital Assistant. On the other hand, small cameras using CMOS sensors may provide a solution for energy consumption; but the traditional CMOS-based small cameras suffer low light sensing performance, which is intrinsic to the nature of CMOS APS sensors caused by shallow junction depth in the silicon substrate and its active transistor circuitry taking away the real estate preciously needed for photo-sensing.
U.S. Pat. Nos. 5,528,043; 5,886,353; 5,998,794 and 6,163,030 are examples of prior art patents utilizing CMOS circuits for imaging. These patents have been licensed to Applicants"" employer. U.S. Pat. No. 5,528,043 describes an X-ray detector utilizing a CMOS sensor array with readout circuits on a single chip. In that example image processing is handled by a separate processor. U.S. Pat. No. 5,886,353 describes a generic pixel architecture using a hydrogenated amorphous silicon layer structure, either p-i-n or p-n or other derivatives, in conjunction with CMOS circuits to for the pixel arrays. U.S. Pat. Nos. 5,998,794 and 6,163,030 describe various ways of making electrical contact to the underlying CMOS circuits in a pixel. All of the above US patents are incorporated herein by reference.
Combining CMOS and MOS sensors with external processors can result in complexity and increase production costs. A need exists for improved image sensor technology which can provide image sensors with cost, quality, performance and size improvements over prior art sensors.
The present invention provides novel MOS or CMOS based photoconductor on active pixel image sensor. Thin layers of semi-conductor material, doped to PIN or NIP photoconducting layers, located above MOS and/or CMOS pixel circuits produce an array of layered photodiodes. Positive and negative charges produced in the layered photodiodes are collected and stored as electrical charges in the MOS and/or CMOS pixel circuits. The present invention also provides additional MOS or CMOS circuits for reading out the charges and for converting the charges into images. With the layered photodiode of each pixel fabricated as continuous layers of charge generating material on top of the MOS and/or CMOS pixel circuits, extremely small pixels are possible with almost 100 percent packing factors. MOS and CMOS fabrication techniques permit sensor fabrication at very low costs. In preferred embodiments all of the sensor circuits are incorporated on or in a single crystalline substrate along with the sensor pixel circuits. Techniques are disclosed for tailoring the spectral response of the sensor for particular applications. For example, in preferred embodiments the spectral range of the sensors can be adapted to cover ultraviolet and near infrared as well as the visible range or any portion of this broad range. Some preferred embodiments of the present invention include additional features to minimize cross talk between pixels. These additional features include a gate bias transistor at each pixel that maintains the charge-collecting element of each pixel at approximately the same electrical potential. In other embodiments semi-conductor material that surrounds charge collecting elements is doped with carbon to increase its electrical resistance. Embodiments provide quantum efficiencies greater than 50 percent and extremely fast response times of a few hundred microseconds or less.
In particular preferred embodiments the sensor is a 0.3 mega-pixel (3.2 mmxc3x972.4 mm, 640xc3x97480) array of 5-micron square pixels that is compatible with a lens of {fraction (1/4.5)} inch optical format. In these preferred embodiments the sensor along with focusing optics is incorporated into a cellular phone camera to permit transmission of visual images along with the voice communication. The result is an extremely low cost camera at high volume production that can be made extremely small (e.g., smaller than the human eye). High volume production costs for the above 0.3 mega-pixel camera are projected to be less than $10 per camera. Preferred embodiments also include infrared and ultraviolet cameras with high quantum efficiencies and fast response times and large pixel arrays such as a 2 million-pixel high definition television format sensor.